(1) Field of the Invention
The present invention relates to an image forming device and an image forming method for feeding sheets contained in a container one by one, and forming an image on each of the sheets that have been fed.
(2) Description of the Related Art
An image forming device (a printer, etc.) forms an image on an image carrier such as a photosensitive drum. In parallel with this operation, the image forming device causes (i) a pickup roller to pick up sheets contained in a feed cassette, (ii) a feed roller and a separator roller that is pressed against the feed roller to individually separate the sheets that have been picked up, and (iii) each of the sheets that have been individually separated to be transported via a pair of transportation rollers to a pair of abeyant registration rollers.
Although the pair of registration rollers are abeyant, the feed roller and the pair of transportation rollers keep rotating. Hence, a front end of each sheet in a transportation direction comes to a halt while being stuck in between the pair of registration rollers, whereas a rear end of the sheet is pushed along the transportation direction by the feed roller and the pair of transportation rollers. As a result, a certain curve is formed in the sheet in the vicinity of its front end. This curve is formed to correct a skew of the sheet.
Rotation of the pair of registration rollers is started at a predetermined time, due to which the following times coincide with each other: (i) a time at which the front end of the sheet arrives at a transfer position on the image carrier after the rotation of the pair of registration rollers has been started; and (ii) a time at which a front end of the image formed on the image carrier arrives at the transfer position. Due to the rotation of the pair of registration rollers, the sheet is transported toward the transfer position. In the transfer position, a transfer roller is pressed against the image carrier. While the sheet is held between and transported by the image carrier and the transfer roller in the transfer position, the image on the image carrier is transferred onto the sheet with the front end of the image and the front end of the sheet aligned with each other.
In the field of image forming devices, there has been strong demand for image forming devices that are smaller in size and require less manufacturing costs. One way to meet such demand is to structure an image forming device in which a feed roller and a pair of registration rollers are arranged on a transportation path. This image forming device is different from the above-described image forming device in which the feed roller, the pair of transportation rollers and the pair of registration rollers are arranged in this order on the transportation path. As the image forming device with no pair of transportation rollers requires a shorter transportation path, it can be manufactured smaller in size. Furthermore, as this image forming device does not include the pair of transportation rollers, it can be manufactured at lower manufacturing costs as well.
However, the image forming device with no pair of transportation rollers may cause delay in transportation of a sheet depending on a size of the sheet, for the following reason.
When a sheet is being transported, the feed roller rotates along with the pair of registration rollers to facilitate the transportation of the sheet. However, the rotation of the feed roller is suspended shortly before the rear end of the sheet passes the feed roller. The purpose of thus suspending the rotation of the feed roller during the transportation of the sheet is to prevent multiple transportation whereby more than one sheets are transported at a time.
More specifically, if the feed roller keeps rotating after the rear end of a first sheet has passed the feed roller, then there is a higher chance that the feed roller feeds a second sheet, which had been separated from the first sheet by the separator roller, with almost no gap between the rear end of the first sheet and the front end of the second sheet (i.e., multiple transportation). In order to prevent the first and second sheets from being fed in close succession, rotation of the feed roller is mandatorily suspended when a certain portion of the first sheet passes the feed roller, the certain portion being away from the rear end of the first sheet by a predetermined distance L (approximately 10 [mm]) in the transportation direction.
Once the rotation of the feed roller has been suspended, transportation load (a brake) is applied to the rear end of the first sheet, the transportation load originating from the force of pressure between the feed roller and the separator roller. At the same time, the front end of the first sheet is pulled toward the transportation direction due to rotation of the pair of registration rollers. Consequently, the curve formed in the first sheet is released, and the first sheet is stretched to full length. At this moment, the largest transportation load is applied to the first sheet.
Assume a case where the image forming device with no pair of transportation rollers uses a sheet A having a first size whose length in the transportation direction is longer than a length (M+L), where M is a distance from the position of the feed roller to the transfer position on the transportation path, and L is the aforementioned distance L. In this case, during transportation of the sheet A, the front end of the sheet A in the transportation direction is located past the transfer position by the time the rotation of the feed roller is suspended. After the rotation of the feed roller has been suspended, the sheet A is transported by both of the pair of registration rollers and the transfer roller; therefore, the sheet A rarely slips when being held between the pair of registration rollers, even if some degree of transportation load is applied to the sheet A.
In contrast, assume a case where the image forming device with no pair of transportation rollers uses a sheet B having a second size whose length in the transportation direction is shorter than the aforementioned length (M+L). In this case, during transportation of the sheet B, the front end of the sheet B in the transportation direction is yet to arrive at the transfer position by the time the rotation of the feed roller is suspended. After the rotation of the feed roller has been suspended, the sheet B is transported only by the pair of registration rollers. In this case, transportation load is applied only to the pair of registration rollers. The moment the transportation load is also applied to the sheet B (especially, the moment the largest transportation load is applied to the sheet B due to release of the curve formed in the sheet B), the rear end of the sheet B is pulled strongly toward a direction opposite to the transportation direction. This may cause the sheet B to slip through the pair of registration rollers.
When this sheet slip occurs, the front end of the sheet B arrives at the transfer position with delay that is equivalent to an amount of the sheet slip. Thus, in the transfer position, the front end of the sheet B is misaligned with the front end of the image in the transportation direction. As a result, after the image has been transferred onto the sheet B, the image is printed on the sheet B in such a way that the image is misaligned with the sheet B.